1. Field of the Invention
The present invention relates to an OLED panel having an organic OLED (OLED: Organic Light Emitting Diode) formed on a substrate and sealed between the substrate and a cover member, and also to an OLED module mounting an IC or the like including a controller on such an OLED panel. In the present description, both an OLED panel and an OLED module are, collectively, referred to as a light-emitting device. The present invention, furthermore, relates to a driving method to a light-emitting device and an electronic apparatus using such a light-emitting device.
2. Description of the Related Art
The OLED, spontaneous to emit light, provides high visibility but does not require such a backlight as needed on a liquid-crystal display (LCD) thus optimally reducing the thickness, which is furthermore limitless in viewing angle. Consequently, the light-emitting devices using OLEDs have recently drawn attentions as the display devices taking the place of CRTs and LCDs.
The OLED has a layer containing an organic compound (organic light-emitting material) to cause electroluminescence under the application of an electric field (hereinafter, described as organic light-emitting layer), an anode and a cathode. The electroluminescence on an organic compound includes the emission of light of upon returning from a singlet excitation state into the ground state (fluorescence) and the emission of light of upon returning from a triplet excitation state into the ground state (phosphorescence). The light-emitting device of the present invention may use either one or both of such emission of light.
In the description, every layer provided between an OLED cathode and an anode is defined as an organic light-emitting layer. The organic light-emitting layer, concretely, includes a light-emitting layer, a hole injection layer, an electron injection layer, a hole transport layer and an electron transport layer. Basically, the OLED has a structure layered with an anode, a light-emitting layer and a cathode in the order. In addition to this structure, some structures possess an anode, a hole injection layer, a light-emitting layer and a cathode or an anode, a hole injection layer, a light-emitting layer, an electron transport layer and a cathode in the order.
It is problematic in placing the light-emitting device into practical application that the brightness of an OLED lowers due to deterioration in the organic light-emitting material.
The organic light-emitting material is less resistive to moisture, oxygen, light and heat, and to be acceleratingly deteriorated by them. Specifically, the rate of deterioration is dependent upon a device structure for driving the light-emitting device, organic light-emitting material properties, electrode materials, conditions in a fabrication process, a driving scheme to the light-emitting device and so on.
With even a constant voltage to the organic light-emitting layer, if the organic light-emitting layer deteriorates, OLED brightness lowers. This results in obscured image display. Note that, in the description, the voltage applied from a pair of electrodes to an organic light-emitting layer is defined as an OLED drive voltage (Vel).
Meanwhile, in a color display scheme using three kinds of OLEDs corresponding to R (red), G (green) and B (blue), the organic light-emitting materials forming the organic light-emitting layer are different between the colors to which the OLEDs correspond. Consequently, there is a possibility that the OLED organic light-emitting layer deteriorates at a different rate dependently upon the color. In this case, as time elapses the brightness of OLED becomes different by the color, making impossible for the light-emitting device to display an image with a desired color.
Meanwhile, the temperature of the organic light-emitting layer relies upon the temperature of outside air or the heat generated by the OLED panel itself. However, the OLED generally has a flowing current value varying with temperature. FIG. 27 shows a change of a voltage-current characteristic of an OLED when changing the temperature of the organic light-emitting layer. At a constant voltage, when the temperature of the organic light-emitting layer increases, the OLED drive current increases. Because the OLED drive current and the OLED brightness are in a proportional relationship, the brightness on the OLED increases with the increase in the OLED drive current. In this manner, because OLED brightness varies with the temperature of the organic light-emitting layer, display is difficult at a desired gray scale. The consumption current of the light-emitting device increases with the rise of temperature.
Furthermore, because generally the change rate of OLED drive current against temperature change is different depending on the kind of an organic light-emitting material, there is a possibility in color display that the OLED brightness of each color dependently varies with temperature. The brightness balance, if disordered between the colors, makes impossible to display in a desired color.
It is an object of the present invention to provide a light-emitting device capable of obtaining a constant brightness regardless of deterioration in an organic light-emitting layer or temperature change and further of providing display with a desired color.